What file must I use for a animation render to post online?
Although there are plenty of formats that are currently supported by Facebook, when uploading to the online video sites, I personally like to choose H.264 format, and simply choose the same in the Preset list in Encoding panel after that. Then give it a tweak if you want. The result format is *.avi in this case. Of course you can also try Xvid and others, as long as  they can be well supported by those sites. FYI and good luck.